The invention is directed to a strongly acid bath for the electrolytic deposition of gold alloy coatings having a low content of platinum metals.
For the gold plating of low current contacts preferably there are employed in the electro arts electrolytic gold baths whose gloss and wear resistance are produced by the addition of small amounts of soluble salts of the iron metals, iron, cobalt and nickel. Such gold baths are operated mostly in a citrate buffer (pH 3-6) having a gold content between 1 and 12 g/l as potassium dicyanoaurate (I) as well as soluble salts of iron, cobalt and nickel in concentration of 50 mg to 5 g/l. These coatings deposit up to 10 microns thick in glossy and crack free form and are wear resistant. Disadvantageous is the fact that with the coatings produced from such baths the contact resistance of the coating, caused by the non-noble components in the precipitating alloys, increases under unfavorable conditions around several powers of ten compared to pure gold.
This increase of the contact resistance is not observed on electrolytic coatings of palladium, rhodium and platinum. Since such coatings at present cannot be produced crack free in thicknesses as desired or because the price of these metals are uneconomical the use of the platinum metals as contact material has not been able to be carried through. Since it is known from metallurgically produced gold alloys with platinum metals as alloying elements that they exhibit a constant low resistance it has already been tried to deposit gold and platinum metals together from aqueous solution. Thus there is known from U.S. Pat. No. 3,923,612 an electrolytic bath in which gold aurate solutions are heated at pH values of around 14 with small amounts of platinum in the form of the hexahydroplatinate. There are obtained from such solutions gold/platinum alloy coatings which do not exhibit the above described disadvantages. The disadvantage of these baths is the expensive production of the gold aurate solution as well as the slight stability against fluctuations in pH in operation.
There are likewise known attempts to codeposit platinum group metals from acidic gold alloy baths in which the gold is present in weakly acid solution (pH 4-6) as dicyanoaurate (I) (Korbelak U.S. Pat. No. 3,893,896 the entire disclosure of which is hereby incorporated by reference and relied upon), but in these baths only the very slightest amounts of platinum metals can be deposited together with the gold. The necessary hardness and therewith the necessary wear resistance for contact gold coatings accordingly are not attained.
Therefore it was the problem of the present invention to develop a strongly acid bath for the electrolytic deposition of gold alloy coatings having low contents of platinum group metals which produce hard, abrasion resistant coatings having a low, electrical contact resistance which remains constant, is easy to produce and remains stable in operation.